1. Field of the Invention
The present invention relates to a tunnel current probe moving mechanism for moving a large number of tunnel current probes along a substrate surface in order to observe a state of the substrate surface or to write or read desired data in or from the substrate.
2. Description of the Related Art
If a conductive sample is used as a substrate, and a large number of tunnel current probes are moved, i.e., scanned, along the sample, a fine surface pattern of the sample can be observed with a resolution of an atomic level. A tunnel current probe moving mechanism for such a scanning operation is called a scanning tunneling microscope (to be referred to as an STM hereinafter) and is developed by Binning et al. in IBM. The STM will be described below.
It is known that when a probe with a sharp point having a curvature radius of several 100 nm is caused to approach a surface of a conductive sample up to a distance of about 1 nm, and a predetermined voltage is applied between the probe and the sample, a tunnel current flows therebetween. This tunnel current is highly sensitive to a change in distance between the sample and the probe. With a distance change corresponding to, e.g., one atom, the tunnel current changed ten times or more in value. A microscope for observing a sample surface by using such properties of a tunnel current is a so-called STM. Assume that a probe is mounted on a three-dimensional driving member capable of moving in a three-dimensional direction. If this probe is two-dimensionally scanned on a sample surface in the X-Y direction while it is servo-controlled in a direction (Z-axis direction) perpendicular to the sample surface (X-Y plane) so as to keep the tunnel current constant, the probe traces irregularity of the sample surface with the distance to the sample surface being kept constant. A servo control amount of the probe in the Z-axis direction at this time is extracted in synchronism with a scanning signal in the X-Y direction, and the position of the distal end of the probe is output as a three-dimensional image, thereby observing the fine surface pattern of the sample as an image at the atomic level.
Note that if an observation surface of a sample is flat at the atomic level, a means may be used, which is designed to image a tunnel current detected when two-dimensional scanning of a probe is simply performed in the X-Y direction without performing the above-mentioned servo control in the Z direction.
When a small region on a sample surface is to be observed by the STM, according to a conventional method, the sample is observed first with an optical microscope to specify a required observation portion, and this portion is then observed in detail with the STM.
If, however, the optical microscope is to use an object optical system having a magnification of X50 to X100, which is frequently used, the operation range, i.e., the distance between the sample surface and the object lens is normally set to be several millimeters (about 5 mm at maximum). Since STMs using conventional tripod type and tube scanner type three-dimensional driving members are large in size, they cannot be inserted in the operation range. For every observation, therefore, the objective lens of the optical microscope and the probe unit of the STM must be exchanged. As a result, an observation position is shifted or a cumbersome operation is required.
Another problem of the conventional STM is associated with relative vibrations of a sample and a probe.
Generally, floor vibration having an amplitude of about 1 .mu.m and mainly constituted by frequency components of 100 Hz or less is observed even at a place where no vibration source exists near. This floor vibration causes the relative vibrations of a sample and a probe of the STM and appears as noise in an obtained STM image. As is well known, antivibration performance can be improved by softening an antivibration system and increasing the rigidity of the overall apparatus. For this reason, in the STM, the resonance frequency of an antivibration base must be decreased as much as possible, and that of the STM unit must be increased as much as possible (normally, 10 kHz). In the STM using the conventional tripod or tube scanner type three-dimensional driving member, however, since a probe scanning system for scanning a probe and a sample supporting system for supporting a sample are generally constituted as independent units, the rigidity of the overall apparatus is inevitably decreased. Since these two systems cannot constitute a resonance system having several 100 Hz, resonance easily occurs. That is, as described above, noise is generated in an STM image, and it is difficult to obtain an image having a high resolution at the atomic level. In order to solve the above-described two problems, C. F. Quate et al., Stanford University have developed a technique of forming a cantilever type STM (to be referred to as a micro STM hereinafter) having a size of 1,000 .mu.m.times.200 .mu.m.times.5 .mu.m on a silicon substrate by using a microfabrication technique similar to the IC process. FIG. 1 is a perspective view showing a schematic arrangement of this micro STM. A cantilever 90 comprises a piezoelectric members (ZnO) 92 and 93 formed to sandwich an Al electrode 91, and strip-like Al electrodes 94, 95, 96, and 97 arranged in parallel in the longitudinal direction of the cantilever 90. The proximal end portion of the cantilever 90 is fixed to a silicon substrate. In addition, a probe 98 extends from a middle portion of the distal end portion of the cantilever 90. The cantilever 90 is connected to a tunnel current detector (not shown) through a wire (not shown).
When a sample is to be observed with such an arrangement, the silicon substrate surface of the micro STM is urged/fixed against/to the sample surface, and the probe is scanned by the above-described cantilever 90 to detect a tunnel current.
The principle of probe scanning of the micro STM developed at Stanford University will be described below with reference to FIG. 1.
If voltages ar applied to the electrodes 94 and 91, an electric field E4 is generated to be directed from the electrode 94 to the electrode 91. In this case, a portion of the piezoelectric member 92 which is sandwiched between the electrodes 94 and 91 expands in the positive X-axis direction shown in FIG. 1. This can be equally applied to other electrodes. The magnitudes of electric field vectors E1, E2, E3, and E4 shown in FIG. 1 and the scanning directions of the probe 98 have the following relationship:
X: positive direction E1=E2=E3=E4&gt;0 PA1 : negative direction E1=E2=E3=E4&lt;0 PA1 Y: positive direction E1=E2&lt;E3=E4 PA1 : negative direction E1=E2&gt;E3=E4 PA1 Z: positive direction E1=E3&gt;E2=E4 PA1 : negative direction E1=E3&lt;E2=E4
That is, the probe is three-dimensionally driven in the X direction by expanding/contracting the entire cantilever 90. In the Y and Z directions, the probe is driven by expanding one piezoelectric member while contracting the other piezoelectric member so as to distort the entire cantilever 90. As a result, the cantilever 90 is displaced by 2.2 nm/V in the X direction, by 22 nm/V in the Y direction, and by 770 nm/V in the Z direction. By adjusting voltages to be applied to the electrodes in this manner, the probe 98 on the distal end of the cantilever 90 is three-dimensionally scanned.
Such a cantilever type micro STM can be inserted in the operation range of an optical microscope. Therefore, after an observation portion is specified by observing a wide area of a sample surface by the optical microscope, the observation portion can be directly observed in detail with the STM without exchanging the objective lens and the probe unit. In addition, since the cantilever of this micro STM is urged/supported against/on the sample surface, the probe and the sample can be substantially integrated. That is, the resonance frequency between the probe and the sample depends on only the resonance frequency of the cantilever itself, and can be increased to several 10 to 100 kHz. As a result, an STM apparatus having high rigidity can be realized, and an STM image free from the influences of vibrations and having no noise can be obtained.
According to the above-described micro STM developed at Stanford University, however, when the probe is to be moved in, e.g., the Y-axis direction in FIG. 1, the probe is scanned in the form of an arc as the Y displacement of the probe is increased. Therefore, if an irregularity signal obtained by this arcuated scanning is directly output, the obtained image is distorted with respect to the actual sample surface. That is, portions where the probe can be substantially linearly scanned are portions where the Y and Z displacements of the cantilever are close to zero. This greatly reduces the effective scanning range.
Furthermore, the displacement of the cantilever in the three directions, i.e., the X, Y, and Z directions with respect to the same driving voltage varies. For this reason, in order to obtain the same displacement in the X and Y directions, for example, a larger driving voltage is required for displacement in the X direction, and a circuit arrangement is complicated. If the cantilever is driven by the same voltage in the X and Y directions, a displacement in the Y direction becomes smaller than that in the X direction. This elongates the scanning surface of the probe, and hence an elongated STM image is obtained. In order to obtain a substantially square STM image, which is easy to observe, a plurality of times of scanning operations are required.
On the other hand, C. F. Quate et al., Stanford University proposed an apparatus capable of storing data at a density of atomic or molecular level by replacing a sample with a proper recording medium. This apparatus is designed such that a tunnel current probe is arranged on the distal end of a cantilever, which incorporates a piezoelectric driving member and has a length of 1,000 .mu.m, a width of 20 .mu.m and a thickness of 5 .mu.m, by the IC process. With this arrangement, this apparatus is stably operated against external vibrations as an STM.
Subsequently, the present applicants proposed a data memory whose storage capacity is multiplied by increasing the number of tunnel current probes to n to multiply the storage capacity by n times, or by forming a plurality of cantilevers on the same IC substrate.
In general, the storage capacity can be increased by increasing the number of cantilevers on the same substrate. In addition, a data amount which can be stored and reproduced is proportional to the scanning range of a tunnel current probe. This range depends on the length of a cantilever. Therefore, the scanning range of a probe can be increased with an increase in length of a cantilever. However, if the length of a cantilever is increased, the scanning efficiency is degraded as compared with the are of the overall apparatus.
In addition, since the scanning distance is decreased toward the proximal end of a cantilever, only the area of the distal end portion becomes an effective scanning range.
For this reason, the distal end portion of a cantilever may be increased in area so that a large number of tunnel current probes can be arranged. In this method, however, since an operation of the cantilever becomes unstable and the operation stability of the overall apparatus is impaired.